For my project, "mediocre paintings reworked into excellence" (20 to go after this one), once the little color study is complete I start on the larger work.
1. I unframed the painting and hosed off the mediocre work (on Wallis paper so it can take water). I pat it dry with a cloth towel.
2. I squeezed out white and raw umber oil paint onto my glass palette. With a 1" wide brush and odorless mineral spirits, I applied an oil wash foundation (see photo below).
the little color study beside me, I applied white, warm dk gray and dk blue NuPastels. Any other color you see is the "ghost" from the mediocre work I washed off.